Developmental research on ownership understanding has focused on preschool years, with only a limited number of studies with infants. The present study with 19-month-old infants showed that after given information about an agent's ownership of a toy (she claimed 'It's mine!' before grasping the toy), infants, as a group (N = 66, 54.5% female, 77.3% White), accepted that when a new toy was added, the agent could choose either her own toy or the new one. Within the group, however, infants with no siblings (d = 0.494) and those with attachment objects (to a lesser degree; d = 0.525) expected the agent to choose her own toy over the new one. These results highlight the role of these two experiential factors in how infants use ownership information to make predictions about others' choices and thus contribute to the theoretical accounts on the early development of ownership understandings.